1. Field of the Invention
The present invention relates to a battery comprising a cathode, an anode and an electrolyte wherein the capacity of the anode includes a capacity component by insertion and extraction of light metal and a capacity component by precipitation and dissolution of the light metal, and is represented by the sum of them.
2. Description of the Related Art
The development of batteries with a higher energy density has been required according to the downsizing of electronic devices. An example of a secondary battery which can obtain a high energy density is a lithium-ion secondary battery using a material capable of inserting and extracting lithium (Li) such as a carbon material for an anode. The lithium-ion secondary battery is designed so that lithium inserted into an anode material is always in an ion state, so the energy density is highly dependent on the number of lithium ions capable of being inserted into the anode material. Therefore, in the lithium-ion secondary battery, it is expected that when the amount of insertion of lithium is increased, the energy density can be further improved. However, the amount of insertion of graphite, which is considered at present to be a material capable of the most effectively inserting and extracting lithium ions is theoretically limited to 372 mAh per gram on an electricity amount basis, and recently the amount of insertion of graphite has been approaching the limit.
Another example of the secondary battery capable of obtaining a high energy density is a lithium metal secondary battery using lithium metal for an anode, and using only precipitation and dissolution reactions of lithium metal for an anode reaction. The lithium metal has a theoretical electrochemical equivalent of 2054 mAh/cm3, which is 2.5 times larger than that of graphite used in the lithium-ion secondary battery, so the lithium metal secondary battery has the potential to be able to obtain a much higher energy density than the lithium-ion secondary battery. A large number of researchers have been conducting research and development aimed at putting the lithium metal secondary battery to practical use (for example, Lithium Batteries edited by Jean-Paul Gabano, Academic Press, 1983, London, New-York). However, the lithium metal secondary battery has a problem that when a charge-discharge cycle is repeated, a large decline in its discharge capacity occurs, so it is difficult to put the lithium metal secondary battery to practical use. The decline in the capacity occurs because the lithium metal secondary battery uses precipitation-dissolution reactions of the lithium metal in the anode. It is considered that the decline occurs because the precipitated lithium metal is separated from an electrode, or reacts with an electrolyte solution, thereby the lithium metal secondary battery is deactivated.
Therefore, the applicant of the invention have developed a novel secondary battery in which the capacity of the anode includes a capacity component by insertion and extraction of lithium and a capacity component by precipitation and dissolution of lithium metal, and is represented by the sum of them (refer to International Publication No. WO 01/22519). In the secondary battery, a carbon material capable of inserting and extracting lithium ions is used for the anode, and lithium metal is precipitated on a surface of the carbon material during charge. The secondary battery holds promise of improving charge-discharge cycle characteristics while achieving a higher energy density.
However, like the lithium metal secondary battery, the secondary battery uses precipitation-dissolution reactions of lithium, so it is extremely difficult to perfectly prevent separation of the lithium metal from the electrode or a reaction of the lithium metal with the electrolyte solution. Therefore, the secondary battery has a problem that when a charge-discharge cycle is repeated, a larger decline in the discharge capacity occurs, compared to the lithium-ion secondary battery.